Vacuum tube oscillator



P. o. FARNHAM VACUUM TUBE OSCILLATOR May 14, 1935.

Filed Feb. 16, 1952 Patented May 14, 1933 2,001,413 a VACUUM TUBE osommrron Paul 0. Farnham, Boonton, N. J.,

mesne assignments, to Radio Corporation of America, New Delaware assignor, by

York, N. Y.,"a corporation of Application l ebruary 16, 1932, Serial no. 593,335 7 Claims (Cl. 250-36) This invention relates to'vacuum tube oscillators and particularly to oscillators, and superheterodyne receivers including oscillators, adapted to be tuned over a plurality of frequency bands. In multi-range oscillators of the type including a tapped inductance and a tuning condenser, no dificulty is experienced in maintaining oscillation over two or three adjacent frequency bands but the problem-of insuring oscillation becomes increasingly difiicult when additional taps are provided in an attempt to cover more frequency bands. Some of the known types of circuits will oscillate freely over the higher frequency band but will not oscillate satisfactorily, if at all, at substantially lower frequencies. Similarly, other systems will oscillate freely at low frequencies but will not operate satisfactorily at higher fre= quency bands. a

This problem of maintaining oscillation of a plurality of frequency bands is encountered, for example, in the construction of multi-range superheterodynes of the type in which tapped in ductances are employed in the oscillator, the carrier frequency and the intermediate frequency circuits to provide a different intermediate frequency for each carrier frequency band. The in vention will be described as embodied in such a multirange superheterodyne (described and claimed in my Patent No. 1,943,788, dated Jan. 16, 1934 and assigned to the same'assignee as the present application), but it will be apparent that the novel oscillator system may be-employed in other types of circuits or as an oscillation gener= ator of general application.

An object of the invention is-to provide an im-' proved multi-range oscillator of the type includ ing a resonant ,circuit comprising atapped inductance and a tuning condenser; theoscillator circuits being so arranged that the change from one tuning rangeto another requiresno adjustment additional to the selection of the appropriate inductance tap. A further object of the invention is to provide a multi-ra'nge tuned oscil-f lator in which the normal couplingbetween the circuits of the oscillator tube is automatically supplemented by anadditional coupling .when the oscillator is adjusted for operation at that end of the frequency range at which the normal coupling is not sufiicient to maintain oscillation. More specifically, an object is to providefa multirange oscillator in which the plate circuit of the tube is coupled to a portion of the capacity included in the resonant circuit, and inwhich fixed circuit elements provide an inductive coupling between the plate andresonantjcircuits when the tube 1 is" shown oscillator is adjusted for operation at its higher frequency ranges, the coupling effect of the fixed circuit elements being negligible when the oscillator is adjusted for'operation within its lower frequency ranges. Further specific objects relate to the provision of multi-iange superheterodynereceivers including multi range oscillators capable, of maintaining oscillation over an extended range ofirequencies.

These and other objects and advantages of the invention will be apparent from the following specification when taken withthe accompanying drawing in which the single figure is a fragmen-v tary circuit diagram of a superheterodyne' receiver embodying the invention.

In the drawing, the reference numeral l identifies a carrier wave amplifier tube which receives signal energy from a collector structure 2 of any desired form. The amplified tube l is transmittedlto a first detector-tube 3 where it beats with locally generated oscillations from the oscillator tube l to produce a modulated signal of intermediate frequency which is impressed upon anintermediate' frequency amplifier tube 5". This will be recognized as a conventio'nal arrangement of coupled tube circuits in a sup'erheterodyne receiver and one which may be varied, in accordance with the particular design requirements, by theintroduction of additional carrier wave and/or intermediate frequency amplifier stages. 7

In accordance with the present invention, the oscillator system is so constructed that oscillations will be" maintained over an" extended range of frequencies. The adjustment of the superheterodyne for the reception of signalsi-n different carrier frequency bands I changing the eifective inductance of the resonant circuits of the receiver.

The magnitudes of the inductances included in the resonant circuitsof these several types of stages maybe varied by" the use of interchangeable coils or' by tapped coils and switches. For purposes of illustration, tappedcoils and switches are shown and, for this particular typeof adjustable inductance, the coils" preferablytake the form described and claimed in the ccpen'ding applica- Sept. I6, 1931, issued as dated Jan. 29, 1935' and sig'nal energy from is obtained by assignee as the present ap- The input circuit of the carrierwave amplifier as including a tapped inductance L shunted by a' tunin'g condenser C, the switch minute patent. The coupling between the first detector tube 3 and the preceding carrier wave amplifier stage may take the form of a transformer having a secondary substantially identical with the described tapped inductance 'L and switch arm 6, the primary winding, L1 of" the transformer being wound turn for turn ,with its secondary or the inductance Land included, as is customary, in the plate circuit of the last carrier wave amplifier stage. Thehigh potential terminals of the transformer are connected by a condenser C1 to reduce the effective resistance of the secondary winding when a portion of the winding is short-circuited. A

The coupling between the first detector 3 and the first intermediate frequency amplifierB may be of the auto-transformer or tuned impedance type and is illustrated as including a. tapped inductance'Lz in the plate circuit of thedetector; a switch arm l being provided for adjusting the effective impedanceof the inductance L2 that is shunted by the trimming condenserfi. Asindicated by the broken lines 9 connecting the several switch arms 6 and switch arm '1, the switch arms are preferably connected mechanically for simul-' taneous adjustment.

With tuned carrier wave and intermediate frequency circuits of, the described types, it will be apparent that the oscillator '4 must be operated over different frequency bands in accordance with the particular frequency band in which the desired signal frequency is to 'be found. As illustrated, the circuit network of the oscillator tube 4 has a tuned grid circuit comprising a tapped inductance L4, a series lagging condenser Cs, atuning condenser C which is mechanically connected, as indicated by broken lines i, tothe condensers C, employed for tuning the carrier frequency circuits, and a small condenser C2 in shunt with the tuning condenser C. The switch arm 12 which controls the effective magnitude of the inductance Leis preferably mechanically. connected, as indicated by broken line I3, to the common operator 9 of the switches 6. and I of the carrier and intermediate frequency circuits. As the gang condenser is adjusted for tuning, the presence of the series condenser in the tuned circuit of the oscillator makes the value of the total capacity tuning the oscillator coil lag behind the value of the capacity tuning the radio frequency circuits, and the series condenser which thus makes it possible to employ a single tuning control inv superheterodyne receivers will therefore be designated, in the following specification and claims, as the lagging condenser. This will be recognized as one of the known arrangements for tracking the carrier and oscillator circuits to maintain a constant frequency difierencebetween the resonant frequencies of the circuits which are simultaneously tuned by a gang condenser, but it will be ap parent that other known methods of maintaining tracking may be employed.

The oscillator is of the general type described and claimed in my copending application Ser. No. 585,592, filed Jan. 8, 1932, the plate circuit being coupled across the series lagging condenser C5 by a condenser C3, the lead to the-source of' plate current supply includinga choke 14 which'has sufficient impedance to maintain the oscillatory frequency ranges. vention, the normal coupling between the plate potential on the plate of tube 4 substantially the same as that of the junction of condenser Cs and inductance L4. A grid leak resistance R, is connected between the grid and cathode to determinethe direct current potential of the grid.

The oscillatory voltage developed by tube .4 is impressed on the first detector tube 3 by connectingtheplate of tube 4 to the cathode of tube 3 through the blocking condenser 5. The catho,de ofthe detector tube'3 is connected to ground through parallel'paths provided, respectively, by

a condenser l6 and a serially connected choke l1 and bias resistor IS. The choke i1 is included to prevent the resistor I8 from varying the effective ratio of the impedancesof. condensers l5 and [6 as the receiver is tuned over a wide range of frequencies. V 7

So far -as described; the oscillator circuit is constructed in accordance with the disclosure of my just mentioned patent and will function efficiently over'a plurality of frequency bands and, as' described therein, the oscillatory voltage developed in anyone frequency band will increase with decreasing frequency. When, as illustrated, the several inductances are tapped to permit tun .ing over five different frequency bands, it is difficult or impossible to construct an oscillator network which will provide sufiicient coupling to insure oscillation at the highest or two'higher In accordance 'with this inand tuned circuits which-provides for oscillation in the lower frequency bands is supplemented by an inductive coupling M which becomes effective only when the switching system is adjusted for tuning the oscillator of its highest or two higher frequency bands. This inductive coupling is provided by the inclusion in the plate circuit of a fewturns L5 which are coupledto the highest frequency section La'of the tapped inductance L4, the sense of the coupling being such as to increase the coupling between the plate circuit and the tuned grid circuit.

Within all frequency ranges except the highest range or: the two higher rangea the ratio of the turns of inductance L5 to the effective turns of the inductance L4 is so small that the inductive coupling between the plate and grid circuits is negligible. However, the adjustment of the switching system to short circuit all but the highest-frequency section of coil L4 automatically provides the additional coupling which is required for maintaining oscillation over the highest band of frequencies. In one embodiment of the invention, the oscillator was designed to operate over a frequency range of from 225 to 24,000 kilocycles and the two higher frequency bands extended from 4,000 to 9,000 and from 10,000 to 24,000 kilocycles, respectively. The high'frequency section La. of the oscillator inductance L4. was a solenoid winding of four turns and the inductance Ls was a winding of two turns. The supplemental coupling introduced atv the higher frequency rangesinsured oscillation over the highest frequency range and the high frequency-end of the next lower frequency range.

'It will be apparent that the invention is not limited to the particular circuit illustrated in the drawing as other arrangements for automatically introducing an additional coupling at one end of theflfrequency range of a multirange oscillator will be apparent to those familiar with the design and construction ofoscillator circuits.

I claimz-I '1. In an oscillator, the combination with a tube having plate and grid elements cooperating with a cathode; of a circuit between the cathode and one of said elements; said circuit including circuit elements having reactances of opposite types, means for adjusting the effective magnitude of one type of reactive circuit element to tune the circuit over a frequency band, and means for varying the effective magnitude of a circuit element having reactance of the opposite type to adapt the circuit for tuning over different frequency bands; a circuit between said cathode and the other element, said second circuit being coupled across a portion of one of said types of reactive circuit elements, and fixed means providing a supplemental coupling between said circuits which becomes automatically more effective when the effective magnitude of said circuit element having reactance of the opposite sign is varied to one limit of its range of variation in magnitude.

2. In an oscillator, a tuned circuit including a tapped inductance and a tuning condenser, 21. second circuit capacitively coupled to said tuned circuit, and means operative only when the tapped inductance is adjusted for resonating at the highest frequency bands to provide a second coupling of substantial magnitude for supplementing said first coupling.

3. In an oscillator, the combination with a vacuum tube having plate and grid elements cooperating with a cathode; a resonant'circuit between cathode and grid comprising a tapped inductance, a series lagging condenser and a tuning condenser, the junction of said condensers being connected to the cathode; and means coupling the plate circuit across the series lagging condenser; of a coil in the plate circuit and coupled to the section of said tapped inductance adjacent the grid.

4. In a multi-range superheterodyne receiver, an oscillator comprising a tube, a tuned. circuit including a tapped inductance and a tuning condenser, a second oscillator circuit capacitively coupled to said tuned circuit, and fixed means automatically providing a second coupling between said circuits which becomes automatically more effective when the tapped inductance is adjusted for operation over its highest frequency range.

5. An oscillator of the type adapted to be tuned over a plurality of frequency bands and having a tuning condenser cooperating with an inductance divided into sections by a plurality of taps, means for progressively short circuiting any desired number of sections of said inductance except that terminal section which is resonated over the highest frequency band by the tuning condenser, and a second circuit capacitively coupled to the tuned circuit comprising the tuning condenser and tapped inductance, said second circuit including an inductance coupled to the said terminal section of the tapped inductance.

6. An oscillator as claimed in claim 5, wherein' said tuned circuit includes a relatively fixed condenser in series with said tuning condenser and tapped inductance, and said second circuit is coupled across saidfixed condenser.

7. In a vacuum tube oscillator of a type including a pair of circuits, reactive means providing coupling between said circuits and supplemental means supplying additional coupling between said circuits, one of said circuits having a reactive circuit element adjustable in discrete steps to adapt the oscillator for tuning over a plurality of frequency ranges, the method of producing sustained oscillations in certain of said frequency ranges within which the reactive means is not operative to sustain oscillations, which method comprises increasing the energy feed-back through said supplemental coupling means when said oscillator is adjusted to permit operation within said certain frequency ranges, and automatically decreasing the energy feedbackthrough said supplemental coupling means to a negligible quantity when said oscillator is adjusted to permit operation in other frequency ranges.-

- PAUL O. FARNHAM. 

